a) Field of the Invention
This invention relates to the technical field of optical image acquisition.
This invention is directed to a method for coherence tomography with increased transverse resolution based on registering the position of locations diffusely reflecting light along a measurement light beam of a short coherence interferometer, which measurement light beam scans the object and penetrates into the object, wherein the respective position of the locations which diffusely reflect light along the measurement light beam in the object is determined as the path length required in the reference arm for achieving interference with a reference light beam, wherein the measurement light beam is focused for optimizing the transverse resolution in the respective location in the object diffusely reflecting light and this focus remains coherent relative to the reference light during its movement along the measurement path, wherein the required movement of this focus and the path length balancing which is required to ensure its coherence relative to the reference light are brought about by the movement of an individual optical structural component part.
b) Description of the Related Art
In optical coherence tomography (Huang, D.; Swanson, E. A; Lin, C. P.; Schuman, J. S.; Stinson, W. G.; Chang, W.; Hee, M. R.; Flotte, T.; Gregory, K; Puliafito, C. A; Fujimoto, J. G., Optical Coherence Tomography, Science 254 (1991), pages 1178-1181), two-dimensional sectional images of objects are obtained in that the objects are scanned along a line by a light beam. This line gives the transverse dimension of the tomographic image or tomogram. In every position along this line, the radiation also penetrates into the depth of the object. The depth position of points which diffusely reflect light is measured by means of a short coherence interferometer and gives the longitudinal dimension of the tomographic image. The strength of the diffuse reflectance of light from these points determines the brightness in the image points of the tomogram.
In short coherence interferometry, light of low temporal coherence but high spatial coherence is used. The depth position of points diffusely reflecting light is given from the length of the reference beam of the short coherence interferometer. The length of the reference beam is continuously changed; the interference occurring at the interferometer output belongs to object points in the measurement light beam for which there is equality of lengths of the reference beam and measurement light beam calculated up to the object point in question. The known length of the reference beam is then--within the coherence length of the utilized light--equal to the sought for measurement light beam length to the point in the measurement object which diffusely reflects light. This method is known as a "coherence scan".
During the coherence scan, either the measurement object is located in the measurement arm of a two-beam interferometer and the measurement light beam lengths are determined from the optical length of the reference arm of the interferometer (Swanson, E. A.; Huang, D.; Hee, M. R.; Fujimoto, J. G.; Lin, C. P.; Puliafito, C. A.; High-speed optical coherence domain reflectometry, Opt Lett 17 (1992), pages 151-153) or the measurement object is illuminated by a dual beam (see Fercher, A. F.; Li, H. C.; Hitzenberger, C. K.; Slit Lamp Laser Doppler Interferometer, Lasers Surg. Med. 13 (1993), pages 447-542) which exits from a two-beam interferometer and the measurement beam lengths are determined by adapting the path differences in said two-beam interferometer to the optical distances between object points in the measurement object which reflect light diffusely.
Both methods have in common that the longitudinal depth information is obtained by means of a method of short coherence interferometry, but the transverse information is obtained through the scanning process with the measurement light beam. In this respect, a very high optical resolution (=shortest distance between two points which are still separately detectable) is achieved by means of short coherence interferometry in the longitudinal direction, that is, in the direction of the illuminating light beam. This is approximately the magnitude of the coherence length I.sub.c =.lambda..sup.2 /.DELTA..lambda. (.lambda.=wavelength, .DELTA..lambda.=bandwidth of the utilized light), that is, the magnitude of several .mu.m. However, a similarly good optical resolution is achieved transverse to the illumination direction only in exceptional cases; in particular, the transverse optical resolution is not uniformly good over the entire object depth. A high and uniformly good resolution over the entire object depth is achieved by using a dynamic coherent focus, as it is called. By dynamic coherent focus is meant the focus of a light bundle which always remains coherent relative to the reference light even with spatial displacement. There are already proposed realizations of a dynamic coherent focus. The method according to the invention will be explained more fully hereinafter with reference to the Figures.
Focused light bundles have already been used for a long time for precise determination of position and object positioning. However, this still relates to the determination of the position of object surfaces and not to the determination of the depth structure of the objects. For example, the German Offenlegungsschrift DE 2 333 281 (date of publication: Jan. 17, 1974) describes a method for adjusting the focal point of an optical system based on interferometry (but not short coherence interferometry). In this case, the measurement light beam is focused on the object surface and the reflected light is interfered with a reference light beam. The shape of the interference fringes then forms a criterion as to whether or not the object surface is located in the focus. This method is therefore suitable only for determining the position of individual object surfaces and therefore does not represent a direct alternative to the method according to the invention. Further, when depth structure is present, there occur statistical interference phenomena, so-called speckle, which largely defy interpretation. Another method for determining the position of object surfaces is described in U.S. Pat. No. 4,589,773 (patent date: May 20, 1986). In this case, the object surface is obliquely illuminated by a measurement light bundle as in the known optical light section microscope. A longitudinal displacement of the object accordingly results in a transverse displacement of the light spot on the object surface. This light spot is imaged on a special photodetector which converts the amount of deviation of the light spot from the reference position into an electrical signal and thus allows the position of the object to be determined. This method is also suitable only for determining the position of individual surfaces, but not for recording the depth structure of an object. It does not work by interferometry and, therefore, also has no interferometric sensitivity and can therefore not be compared with the method according to the invention.
Further methods for determining the position of individual object surfaces are known in the context of the problem of focusing in compact discs; for example, U.S. Pat. No. 4,631,395 (patent date: Dec. 23, 1986) and U.S. Pat. No. 4,866,262 (patent date: Sep. 12, 1989). These methods are also only suitable for determining the position of individual surfaces but not for recording the depth structure of an object; they do not work by interferometry and therefore cannot be compared with the method according to the invention.
On the other hand, the problem of transverse resolution in coherence biometry and coherence tomography is addressed in the international PCT application WO 92/19930 "Method and Apparatus for Optical Imaging and Measurement" (priority date: Apr. 29, 1991; inventors: Huang, D.; Fujimoto, J. G.; Puliafito, C. A.; Lin, C. P.; Schuman, J. S.). In this patent, the above-mentioned problem of a high and uniformly good transverse resolution over the entire object depth is achieved in that the deflecting mirror is moved synchronously in the reference beam path simultaneous with the movement of the measurement focus.
While a synchronous movement of the measurement focus and deflecting mirror in the reference beam can be carried out technically, it represents considerable additional mechanical and electronic expenditure. Further, the geometric displacement of the measurement focus will generally not correspond to the change in the optical length in the reference beam because there are different indexes of refraction in the measurement light beam path and in the reference beam path. Therefore, the suggestion is found in scientific literature that the focal displacement and the balancing of the optical lengths between the measurement light beam and reference beam are carried out by means of the displacement of an individual optical element; see, for example, Fercher, A. F., Optical Coherence Tomography, J. Biomed. Opt. 1 (1996), no. 2, pages 157-173.
This and the method according to the invention will be described with reference to the following Figures.